1. Field of the Invention
The present invention relates to coating metallic workpieces by electrodeposition; coating of vehicle parts, etc.; and more specifically to a method of providing a uniform film in coating workpieces by electrodeposition coating.
2. Description of the Prior Art
Conventional methods for applying by electrodeposition a coating to a plurality of workpieces being conveyed in a continuous process typically call for placing electrode plates at both the right and left sides or, as the case may be, at the bottom of an electrodeposition bath that contains the paint, bringing in the workpieces to be coated from one side of the electrodeposition bath into the paint, while applying direct current voltage between the workpieces and the electrode plates; and bringing out the workpieces from the other side of the electrodeposition bath. (For examples, JP-B-SHO 58-10476, JP-A-SHO 54-112949 and Japanese Utility Model Publication SHO 58-701)
Such a conventional method for electrodeposition coating, however, presents a problem in that it creates an uneven thickness electrodeposition film of between the workpieces, particularly when the workpieces to be coated are relatively small; for instance, in cases where workpieces are smaller than a vehicle body, such as vehicle parts. For the purpose of improving production efficiency, typically a plurality of workpieces are vertically hung from a conveying means (for example, a suspension hanger), and conveyed in such a configuration through the electrodeposition bath. (For example, Japanese Utility Model Publication SHO 58-701 and Japanese Utility Model Publication SHO 58-4928) When utilizing such a conveying method, among the vertically hung workpieces, the workpiece in the lower position is immersed in the paint contained in the electrodeposition bath first, while the workpiece in the upper position comes out of the paint first. Therefore, the length of time that the workpieces in the lower position stay in the paint, and the length of time that the current flows to the workpieces in the lower position, is longer than that for the workpieces in the upper position; thus, as a consequence, the electrodeposition film on the workpieces in the lower position is thicker than that on the workpieces in the upper position. Such irregularity or non-uniformity in the thickness of an electrodeposition film occurs, also, in case of a large workpiece, such as the body of a vehicle, where the coating on the lower part of the workpiece is apt to be thicker than that on the upper part.
Thicker electrodeposition film may be advantageous, from the rust resistance point of view, in case of the chassis parts of a vehicle, including driving operation parts and engine supporting parts. However, if the thickness of the electrodeposition film on the workpieces reaches the range between 30 .mu.m and 45 .mu.m, such disadvantages as loosened bolts, caused by deformation of the paint film due to fatigue; and the necessity of increasing clamping torque at the screwing part of the nuts can be expected.
It is deemed possible to apply conventional methods; which have been disclosed as solutions to such problems, such as abnormal adhesion of paint at the time of entrance into the bath, or miscoating of paint causing such defects as pinholes, etc; in order to reduce uneveness or non-uniformity in the thickness of the electrodeposition films between lower and upper workpieces or between the lower and upper parts of a workpiece. Such methods, however, are not in themselves intended to reduce irregularities in the thickness of electrodeposition films. Included in such known methods are 2-stage power conduction system, which calls for applying low voltage at the time of entrance into the bath; and then applying a predetermined high voltage once immersion is completed (JP-A-SHO 58-93894: U.S. Pat No. 4,486,284); and the method which calls for removing the electrode plates at the entrance side of the electrodeposition bath (JP-A-SHO 54-112949). In addition to the above, the method which calls for a shielding plate made of insulating material to be placed on the side of the electrode plate facing the workpiece to be coated at the entrance side of the bath so as to restrain sudden influx of a large amount of current (Japanese Utility Model Publication SHO 51-4307).
Further, JP-A-SHO 59-177398 discloses a method for detecting the position of a workpiece when it has become completely immersed in the paint upon entering the bath, so as to start applying voltage and for detecting the position of the workpiece when the workpiece starts to come out of the bath so as to stop applying voltage. In this manner, workpieces are electrodepositedly coated only in the area where the workpieces are completely immersed, thus essentially preventing the problem of irregularities of the electrodeposition film thickness between the upper and the lower workpieces and/or between the upper and lower parts of the workpiece.
However, the methods disclosed in the above-mentioned JP-A-SHO 58-93894 (U.S. Pat. No. 4,486,284), JP-A-SHO 54-112949 and Japanese Utility Model Publication SHO 51-4307 reduce to some extent differences in the thickness of electrodeposition films incurred at the entrance side of the bath, but are not sufficient to prevent such differences in thickness of films of the lower and upper workpieces at the exit side of the bath, because the voltage is still applied, thus continuing electrodeposition coating, for the period from when the workpiece in the upper position starts to emerge from the paint to the time all the workpieces have completely emerged.
According to the method disclosed in JP-A-SHO 59-177398, an electrodeposition coating is impressed by a single means of voltage application to the workpieces in a submersion area (an area of a bath where a conveyed workpiece is completely submerged in the paint). Therefore, no problem should occur as long as only one workpiece or one vertical line of a plurality of workpieces is present in the submersion area at one time, but if a plurality of workpieces are to be continuously conveyed in short intervals, thus resulting in the presence of multiple workpieces on the conveying route at the same time in the submersion area, the following problem is evident. When a workpiece emerges from the electrodeposition bath, voltage application to the workpiece is suspended for the length of time when the workpiece starts to emerge until the workpiece completely emerges from the paint; but the suspension of the voltage application also affects the succeeding workpieces, which are still in the paint, as there is only one voltage applying means.
As a result electrodeposition coating time for the succeeding workpieces is insufficient, thus bringing about the unfavorable consequence of insufficient thickness of film. Further, since the bus bars are one portion of the means of applying the voltage with a fixed electrical potential along the full length of the conveying route of the workpieces, contact by the brush, which is conveyed together with the workpiece, with the beginning of the bus bar will generate a sudden flow of a large amount of electric current. Such sudden current flow will generate sparks and initiate electrodeposition coating that may cause defects on the electrodeposition film such as rash, pinholes, etc.